Preface: I'm running Windows Vista Ultimate and using a
USB to DVI adapter (lower res).
I turned my computer on this morning, and two of my screens (the ones not using the USB to DVI) were almost completely dark, that is, it looked like the brightness was at about 10%. I verified that the actual screens' brightness was at 100%. My third screen (the one connected via the USB to DVI was at normal brightness.
I searched online, and various forums said that it was probably due to a driver related to one of my start-up programs. The driver or other software put Vista in the state when a secure prompt comes up (partially darkens the screen behind the login window), and Vista never popped out of it.
I suspected it was the USB to DVI adapter, so I unplugged it and restarted my computer. Darkness gone, brightness back on my two main screens. I plugged the USB to DVI in again and restarted it, and the darkness came back. Apparently, the USB to DVI adapter's driver messes with the Vista startup. I found a workaround in the Vista settings, but it required me to disable one of the billion security settings--not sure if it really will make my machine less secure, but if others don't want to disable that setting, they can just remember to make sure the USB to DVI adapter is not plugged in when booting the computer. Once it's booted, plug it in and it'll work just fine.
Here is a link to the site that explains the Vista security setting workaround:
http://www.howtogeek.com/howto/windows-vista/make-user-account-control-uac-stop-blacking-out-the-screen-in-windows-vista/
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